With continued miniaturization of semiconductor integrated circuits to achieve greater component density and smaller units, as in large scale integrated circuits, it is required that high resolution lithographic processes be developed for dense metallization networks. A major factor limiting resolution of micron and submicron resist images in electron beam lithography using single layer thick resist films, is the proximity effect due to electron scattering. As a result, multilayer resist approaches that employ thin imaging layers to eliminate or minimize the problems of proximity effect and having increased resist sensitivity are receiving an increased focus of attention by the research and development areas for submicron lithography. Similarly, in optical lithography using refractive projection mask aligners, multilayer resist processes overcome the problems of resolution and aspect ratio because of depth of field limitations in the case of single layer resist images.
A trilevel resist process for integrated circuit devices, typically, comprises a radiation insensitive polymer base layer, a thin O.sub.2 /RIE resistant barrier layer and a thin E-beam, UV or X-ray resist layer. The polymeric base layer is generally applied by spin-coating on a substrate followed by a curing step and deposition of the O.sub.2 /RIE barrier film which is overcoated with a thin coating of an imaging (resist) layer. The O.sub.2 /RIE barrier films conventionally employed in the prior art are mostly inorganic in nature, such as plasma deposited silicon oxide, vacuum evaporated SiO, sputtered quartz and chemical vapor deposited (CVD) silicon nitride. Further information can be obtained on reference to the Havas et al. U.S. Pat. No. 4,202,914; J. Moran, Pol. Eng. Sci. 20, 1097 (1980); L. Rothman, IBM Technical Disclosure Bulletin 23, 2293 (1980); and P. Grabbe et al. J. Vac. Sci. & Tech. 21, 33 (1982).
One method of employing these multilevel resist (MLR) techniques for forming meallization patterns on devices is commonly denoted by the term "expendable mask method", "lift-off method" or "stencil method", which was initially described in U.S. Pat. No. 2,559,389. Improvements to the basic "lift-off method" have been made, as for example, in U.S. Pat. Nos. 3,849,136; 3,873,361; 4,004,044 and 4,202,914.
In conformance with the foregoing, this "lift-off" method includes the use of organic polymeric material as a base or sub-layer on an integrated circuit substrate and a overlying oxygen RIE resistant layer of a diverse material, such as a metal layer, silicon dioxide, silicon nitride or a polydimethylsiloxane, having openings in a selected pattern. Where polydimethysiloxanes have been proposed as oxygen RIE barrier films, they have been solution coated or cast on the polymeric base layer. See the above noted patent of Franco et al. U.S. Pat. No. 4,004,044, and L. Fried in IBM J. of Res. and Dev. 26, 362 (1982).
After deposition of the oxygen barrier mask, corresponding openings are conventionally formed or transferred in the polymeric base layer by reactive ion etching (RIE) in an oxygen ambient utilizing the overlying barrier film as a dry etch mask. The desired thin film, e.g. metal, to be deposited, is then blanket coated over the structure and on the surface exposed by the openings in the polymeric base layer.
When the polymeric base layer is removed by application of a solvent selective for it, the overlying barrier layer and thin film, above the barrier layer, "lift-off" to leave the thin film deposited in the selected pattern on the substrate.
Although, the conventional inorganic and solution coated polydimethylsiloxane films have been effective masking layers in O.sub.2 /RIE processes and have been employed in multilevel metal process for fabrication of integrated circuit devices, there are some problems encountered with these materials which detract from their consistency of function and thus renders that application less attractive in lithographic processes designed for submicron patterning. For example, coherent thin films of silicon oxide or silicon nitride are formed at relatively high deposition temperatures, e.g. 200.degree.-275.degree. C., which results in debris generation at the organic-inorganic interface, and thus problems of reticulation and shriveling are often encountered. A single major disadvantage of inorganic oxygen etch barrier films is a too frequent occurrence of high defect densities that are above the acceptable limits for high resolution lithographic applications. The solution coated or cast thin films of poly(dimethylsiloxanes) and "glass resins", on the other hand, above the problems of non-uniformity, thickness control and shelf life of polymer solutions due to the presence of a certain amount of reactive groups on the polymer chain.